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Protection and Restoration
although the thermal spray coating does not add any strength to an existing material, it is a quick and economical way to restore dimensions of components. After a
part is coated, subsequent grinding operations are often needed to smooth its surface
and to bring the final dimensions to required tolerances. Li (2010) notes that apart
from dimensional restoration, thermal spray coatings can add some functions to light
metals, such as wear resistance, corrosion resistance, bioactivity, and dielectric properties. Some characteristics of the deposition process, such as splat cooling and successive stacking of splats, lead to creation of coatings with unique microstructures
different from conventional materials.
A typical thermal spray system consists of five subsystems (Fauchais et al., 2014):
1. A high-energy, high-velocity jet generation unit, including a torch, power
supply, gas supply, and associated control systems
2. Coating material preparation and its transformation into a stream of molten
droplets, including the powder size distribution and morphology, and its
injection into a high-energy gas stream
3. Surrounding atmosphere and its controlled parameters including humidity,
low pressure, etc.
4. A substrate material with a properly prepared surface
5. Mechanical equipment for controlling motion of the torch and the substrate
relative to each other, i.e., stand-off distance, velocity, etc.
According to Dorfman (2018), there are some key decisions to be made for salvage
and repair materials, including coating thickness, color match, surface profile after
finishing, ease of finishing, bond strength, deposition rate, and application cost. As
an example of a salvage and repair material, he mentions nickel 5 wt% aluminum
powder, manufactured by a variety of different processes, that may be mechanically clad, chemically clad, or gas- or water-atomized. The choice of a process is
based on the application, considering that similar chemical compositions do not
always guarantee equivalent performance. For instance, Ni 5 wt% Al coatings with
higher oxide levels are harder and more difficult to machine than coatings with
lower oxide levels. If a coating exhibits poor cohesive strength and unmelted particles appear in it, this may result in particle pullout during finishing and finally
in a porous surface finish. In addition, a coating with unreacted aluminum within
its microstructure may be a problem for parts exposed to the environment of corrosive substances (Dorfman, 2018). Characteristics of thermal spray processes have
impact on spray particle parameters and metal alloy particle oxidation; splat formation (including solid–liquid two-phase droplet impact involved in cermet coating
deposition) influence features of pores, coating microstructure, dominant effect of
lamellar structure on coating properties, and reactions of spray particles with light
metal substrates (Li, 2010).
Łatka et al. (2020) describe thermal spray techniques used frequently to obtain
functionally graded coatings, namely atmospheric plasma spraying (APS), suspension plasma spraying (SPS), solution precursor plasma spraying, high-velocity
Protection and Restoration
although the thermal spray coating does not add any strength to an existing material, it is a quick and economical way to restore dimensions of components. After a
part is coated, subsequent grinding operations are often needed to smooth its surface
and to bring the final dimensions to required tolerances. Li (2010) notes that apart
from dimensional restoration, thermal spray coatings can add some functions to light
metals, such as wear resistance, corrosion resistance, bioactivity, and dielectric properties. Some characteristics of the deposition process, such as splat cooling and successive stacking of splats, lead to creation of coatings with unique microstructures
different from conventional materials.
A typical thermal spray system consists of five subsystems (Fauchais et al., 2014):
1. A high-energy, high-velocity jet generation unit, including a torch, power
supply, gas supply, and associated control systems
2. Coating material preparation and its transformation into a stream of molten
droplets, including the powder size distribution and morphology, and its
injection into a high-energy gas stream
3. Surrounding atmosphere and its controlled parameters including humidity,
low pressure, etc.
4. A substrate material with a properly prepared surface
5. Mechanical equipment for controlling motion of the torch and the substrate
relative to each other, i.e., stand-off distance, velocity, etc.
According to Dorfman (2018), there are some key decisions to be made for salvage
and repair materials, including coating thickness, color match, surface profile after
finishing, ease of finishing, bond strength, deposition rate, and application cost. As
an example of a salvage and repair material, he mentions nickel 5 wt% aluminum
powder, manufactured by a variety of different processes, that may be mechanically clad, chemically clad, or gas- or water-atomized. The choice of a process is
based on the application, considering that similar chemical compositions do not
always guarantee equivalent performance. For instance, Ni 5 wt% Al coatings with
higher oxide levels are harder and more difficult to machine than coatings with
lower oxide levels. If a coating exhibits poor cohesive strength and unmelted particles appear in it, this may result in particle pullout during finishing and finally
in a porous surface finish. In addition, a coating with unreacted aluminum within
its microstructure may be a problem for parts exposed to the environment of corrosive substances (Dorfman, 2018). Characteristics of thermal spray processes have
impact on spray particle parameters and metal alloy particle oxidation; splat formation (including solid–liquid two-phase droplet impact involved in cermet coating
deposition) influence features of pores, coating microstructure, dominant effect of
lamellar structure on coating properties, and reactions of spray particles with light
metal substrates (Li, 2010).
Łatka et al. (2020) describe thermal spray techniques used frequently to obtain
functionally graded coatings, namely atmospheric plasma spraying (APS), suspension plasma spraying (SPS), solution precursor plasma spraying, high-velocity